Washington’s Bryce Harper acknowledges the crowd after hitting a solo home run during the first inning of Monday’s game against the Milwaukee Brewers at Nationals Park in Washington.

WASHINGTON — Bryce Harper needed only two pitches to earn himself a curtain call.

Back from the disabled list, the Washington Nationals sparkplug launched a fastball from Yovani Gallardo into the visitors’ bullpen for an opposite-field homer Monday night in the first inning against the Milwaukee Brewers.

Harper raised his right arm and pointed to the fans as he finished his familiar sprint around the bases. After lots of high-fives in the dugout, he returned to the top of the steps and gave a thumbs-up to answer the cheers.

Batting third and playing left field, Harper got the benefit of the doubt on the first pitch from Gallardo, holding up just enough for a check-swing to be called a ball.

There was no doubt about the second pitch. The ball cleared the left-field fence by a few feet, a solo shot for Harper’s 13th homer of the season and 24th RBI.

The 20-year-old slugger was reinstated from the 15-day disabled list a few hours before the game, following his recovery from a knee injury that stemmed from a collision with the Dodger Stadium fence. He had been gone so long that manager Davey Johnson wanted to be sure it wasn’t a mirage when he saw Harper in the clubhouse.

“I went in there and said, `Is that really you? Are you OK? Are you ready to go?”’ Johnson said. “He shook my hand and said, `I’m ready.’ It seemed like I hadn’t seen him in half a year. It seems that long.”

Harper missed 31 games with bursitis, diagnosed after he reinjured his left knee several times after he kept trying to play for two weeks following the mishap in Los Angeles on May 13. He pronounced himself fit after going 4 for 11 with a double, a triple and a home run during a four-game rehab assignment last week with Class-A Potomac and Double-A Harrisburg.

“A little swelling, but not much,” Harper said. “Nothing to be cautious about. I feel good, and it’s to the point where I want to play.”

Harper said he would wear padding to protect the knee.

The Nationals were 25-19 when Harper played, 16-21 when he didn’t. That added up to a middling 41-40 mark at the halfway point for a consensus preseason pick to repeat as NL East champions. He was batting .287 with 12 homers and 23 RBIs when he went on the DL.

“Not only his bat, but I think the energy he brings, he takes a lot of the focus and he’s a competitor,” Johnson said. “Just to have that 20-year-old energy running around 100 percent, I’m sure he’s still probably going to run into the wall, I’m sure he’s going to dive headfirst. ... I don’t want to put a damper on that. That’s who he is, and that’s how he plays the game, and that’s great.”

The return of Harper was the last thing the struggling Brewers needed. Milwaukee arrived in Washington on a five-game losing streak.

“We’ll see after the series how bad of a break it is. ... What it does is it adds a guy who can drive the ball out of the ballpark,” Brewers manager Ron Roenicke said. “He can hit any pitch, and you have to be really careful with how you pitch him.”

Harper’s return puts Johnson closer to the lineup he envisioned at the start of the year, except that Anthony Rendon has replaced the demoted Danny Espinosa at second base. All that’s left is for catcher Wilson Ramos to return from a left hamstring injury. Ramos was scheduled to catch seven innings at Potomac on Monday and could return to the major league club as early as Thursday.

Johnson moved Rendon from second to seventh in the batting order to create a left-right-left-right-left top of the order consisting of Denard Span, Jayson Werth, Harper, Ryan Zimmerman and Adam LaRoche.

“It’s all about matchups late in the ballgame that you want to create,” Johnson said.

NOTES: Johnson said LHP Ross Detwiler had a good bullpen session Monday and is on pace to start Wednesday as scheduled. Detwiler had stiffness in his lower back after his last start. ... The Nationals optioned RHP Erik Davis to Triple-A Syracuse to make room for Harper.